Journal of Conference Abstracts

Session J05:5A

J05 : 5A/01 : G6

Seepage Monitoring on an Embankment Dam with Geophysical Methods

Hans Thunehed (hatu@sb.luth.se)¹ &

Carl-Axel Triumf²

¹ Dept Applied Geophysics, Lulea University of Technology, S-97187 Lulea, Sweden

² Triumf Geophysics AB, Sunderbyv 158, S-95442 S Sunderbyn, Sweden

A system for dam core monitoring based on frequent measurements of the spontaneous potential (SP) and electric resistivity at a large number of points on a dam has been developed. The system was installed in 1993 on the eastern Suorva dam in northern Sweden, where measurements have been carried out more or less continuously since august 1993. The dam is an embankment dam that is 700 m long and 50 m high. The water level in the reservoir varies over the year due to requirements for power production and water inflow.

The measurements are made automatically and the system is controlled remotely via modem. Very large boulders cover the upstream side of the dam and prevent galvanic contact. Thus, measurements are only made on the downstream slope. This part of the dam is exposed to rough climate conditions like low temperatures and drought. The electrodes have therefore been buried at a depth of roughly one m. Lead-leadchloride electrodes were used but due to ageing problems some of them have been replaced by copper-coppersulphate electrodes.

SP data correlate to the water level in the reservoir thus showing that a streaming potential is developed in the dam. Positive SP anomalies on the downstream slope of the dam are expected due to seepage through the dam core. There are several areas on the dam with local positive anomalies that can be interpreted as caused by anomalous seepage. Other causes of SP are however evident like rainfall, drought, soil moisture, freezing and presence of metallic objects in the dam. By studying the change of SP as a function of water level, anomalies caused by other things than seepage can be ruled out. The only component of the SP that is expected to have a positive correlation with the water level is the streaming potential. Maps showing the temporal change of SP, during periods with increasing water level, indicate areas with anomalous seepage. Rough estimates of the depth to the seepage can also be made. The resistivity measurements are not affected by the seepage, but good knowledge of the internal resistivity structure of the dam is required as an aid during interpretation of the SP data.

J05 : 5A/02 : G6

Hydrogeophysical Investigations of old Contaminated Sites: A Combined Approach to Delinate the Extent of Contamination and to Optimize Position and Numbers of Observation Wells (Landfill Riet, Switzerland)

Peter Huggenberger (Huggenberger@ubaclu.unibas.ch)¹,

Remo deIaco² &

Anette Johnson (Johnson@eawag.ch)³

¹ Geological-Paleontological Institute, University of Basle, 4056 Basle, Switzerland

² Institute of Geophysics, Federal Institute of Technology, 8093 Zurich, Switzerland

³ Federal Institute of Environmental Science and Technology, EAWAG, 8600 Duebendorf, Switzerland

Exemplary investigations of a mixed landfill in a gravel and sand aquifer have been carried out in the context of a multidisciplinary research program on the sustainable management of resources. The goal set at the Riet site was the assessment of the type and load of contaminants transported from the landfilll site to the adjacent aquifers. The specific goals addressed in this paper are (1) the delination of the differences in the geochemical conditions of areas with respect to contaminated sites and (2) the evaluation of the potential of geophysical methods with respect to their capabilities of describing shallow subsurface geological heterogeneities. An EM-31 (electrical conductivity) survey was performed with a 2 x 2 m grid space, covering the area of the different waste compartments. These data were used to position the groundwater observation wells and the localities for geochemical sampling. Later, a series of single EM-31 transects connecting the drillholes allowed the verification of the calibration of the EM-31 measurements based on geochemical data. The results show that very small changes of apparent electrical conductivities in the order of 1 to 2 mS/m can be resolved. Inphase mesasurements allow an exact mapping of shallow installations which might be critical for the installation of an optimized observation network. The results also indicate that geophysical methods might significantly contribute for the optimization of positions and numbers of observation wells.

Buselli, Get al, Detection of groundwater contamination near waste disposal sites with transient electromagnetic and electrical methods, in Ward, S. H. (ed.), Geotechnical and Environmental Geophysics, vol 2/5, 27-39, (1990).

Bundesamt für Umwelt, Wald und Landschaft (BUWAL), Altlasten-Konzept für die Schweiz, Herausgegeben vom BUWAL, Bern, Nr. 220, (1994).

Ward, SH, Pridmore, DF, Rijol Glenn WE1974, Multispectral Electromagnetic Exploration of Sulphides. Geophysics, 39/5, 661-666, (1974).

J05 : 5A/03 : G6

ENVIROMT: A New Radio-Controlled Source System and First Results

Laust B Pedersen (lbp@geofys.uu.se)¹,

Lars Dynesius (ld@geofys.uu.se)¹,

Mehrdad Bastani (mba@geofys.uu.se)¹,

Mehran Gharibi (mg@geofys.uu.se)¹ &

Uli Matzander (geo@metronix.de)²

¹ Dep Earth Sciences, Villavägen 16, 752 36 Uppsala, Sweden

² Metronix, Neue Knochenhauerstr 5, D-38100 Braunschweig, Germany

The design of new electromagnetic system using radio frequencies in the band 14 kHz to 500 kHz and two perpendicular horizontal magnetic dipole sources from 1 kHz and upwards has recently been completed by Uppsala University and Metronix company.

Two horizontal electric and three magnetic components are simultaneously measured in two bands 1-25 kHz or 10-500 kHz. In the first band there are numerous VLF transmitters present starting at ca 14 kHz. In the second band, in addition to the VLF transmitters, there are more than 20 transmitters present with a rather uniform distribution of azimuths, allowing for stable estimates of the full impedance tensor and the tipper vector. If only radio frequencies are used each site can be measured in just a few seconds. The system consists of three short magnetic sensors built into one small unit and 4 small steel electrodes connected to buffer amplifiers all of which are fed into a central analogue signal conditioner before being transferred to an advanced signal processor box with AD converters, GPS synchronisation, radio-communication modem between transmitter and receiver and a new dedicated software package for data playback and visualisation, and automatic 1-D inversion.

Results from the crystalline of Sweden are compared with coincident VES measurements over a narrow 200 m wide valley near Stockholm. Both methods define the resistive boundaries of the valley and a small, shallow resistive block in the middle of the valley. Results from a waste dump site in the Netherlands, where a number of geophysical techniques have been applied to trace the leakage of conducting fluids in the form of plumes, show good agreement with standard VES and TEM measurements. 2-D inverse models give a very clear indication of the lateral extension of the plume.

J05 : 5A/04 : G6

Lake Seismic Reflection for Environmental Surveying, a Source Comparison

André Pugin (Pugin@sc2a.unige.ch),

Ira Baster (Baster@sc2a.unige.ch),

Stéphanie Girardclos (Girardcl@sc2a.unige.ch) &

Walter Wildi (Wildi@sc2a.unige.ch)

Institut F.-A. Forel, Rte de Suisse 10, CH-1290 Versoix, Switzerland

In the water cycle, most pollutants are stored in the sediments of rivers, lakes and oceans. Also, they can return into the water as erosion events occur, or during geotechnical work. As a consequence, very high resolution information on the sedimentary geometry is needed in many cases, such as environmental impact studies, construction projects and dredging of harbors and channels. This paper presents a comparison between different sources used in high resolution seismic for the investigation of sedimentary geometry.

Most of the applied seismic reflection geophysical surveys over water surfaces imply that the observation of the first meters of sediment is made in shallow water. In this environment, the presence of natural biogenic gas is a barrier to the acoustic energy. Therefore a seismic source for such investigations requires: a high frequency (the wavelength must be as short as possible), a high energy (a strong energy is required to penetrate in depth).

An airgun, a sparker, and a subbottom 12 kHz echosounder have been tested in a comparative study over Lake Geneva and Lake Zürich with the following parameters: shot in distance with a computer assistance and a DGPS positioning, single- or multichannel acquisition and a digital record of the signal. Digital signal processing gives great advantages for wavelet shape improvements.

The small airgun has the advantage of being a powerful source but the frequency range does not exceed 1.5 kHz. The sparker gives higher frequency responses which are close to the needed resolution with an energy centered at 1 kHz. The 12 kHz echosounder has generally little penetration in regard to the absorption of the high frequencies by the unconsolidated gas bearing sediment. As none of these sources are completely accurate in shallow water environments, a new and light source is being developed to generate a powerful energy in the frequency band of 2 to 3 kHz with an equipment that can fit on very small boats available only on small water surfaces.

J05 : 5A/05 : G6

Investigation of a Glaciofluvial Delta in Switzerland Using High Resolution Reflection Seismic and Georadar

Frank O. Nitsche (frankn@augias.ig.erdw.ethz.ch),

Heinrich Horstmeyer (heinrich@augias.ig.erdw.ethz.ch),

Frank Büker (frankb@augias.ig.erdw.ethz.ch),

Michiel van der Veen (michiel@augias.ig.erdw.ethz.ch) &

Alan G. Green (alan@augias.ig.erdw.ethz.ch)

Inst. of Geophysics, ETH Zürich, ETH Hönggerberg, CH-8093 Zürich, Switzerland

The present shape and sediment infill of alpine valleys in northern Switzerland are features that were mainly created and deposited during the last ice ages. Glacial erosion caused overdeeping of these valleys. During several stages of retreat and advance of the ice, the valleys were filled with heterogeneous deposits of tills and glaciofluvial and lake sediments. Today these valleys are the sites of landfills and ground water reservoirs as well as sources of gravel and sand. Here, we present the results of geophysical surveys conducted across an area of the Reuss Delta on the southern shore of Lake Lucerne. This area is typical of the type of valleys described above. During the past few years the ETH Institute of Geophysics has carried out several high- resolution seismic reflection surveys within this valley. Seismic sections perpendicular to the valley axis provided a general cross-section on which we interpreted strong V-shaped reflections at 200-550 m depth as the base of sediment deposition. Using information from a nearby 300 m-deep borehole, we identified the main units in the seismic sections as a thick succession of lacustrine clays and fluvial sand and gravel deposits. A seismic section near the lake shore represented a complex shallow system (100-150 m depth) of delta progression and braided channel interaction. Unfortunately, coherent noise (e.g. ground roll and refracted waves) did not allow reliable identification of reflections in the uppermost 20 m. To obtain more information about this part of the shallow subsurface we have collected several georadar profiles. The radar signal penetrated down to a depth of 12 m, which was not deep enough to overlap with the useful part of the seismic data. On the georadar profiles we identified different facies and channel-like features. In addition, a 3D-georadar data set (40x80 m) provided more detailed information of one of these channels. Although we could not tie the georadar data directly into the seismic sections, combining both data sets allowed us a better understanding of the evolution of the Reuss Delta.

J05 : 5A/06 : G6

Estimating Permeability of Fractured Rock Using Microseismicity: Application to the 3D Modeling of Fluid Flow Transfers at the Geothermal Soultz Site

Pascal Audigane (audigane@crpg.cnrs-nancy.fr)¹,

Sergei Shapiro (shapiro@ensg.u-nancy.fr)² &

Jean-Jacques Royer (royer@crpg.cnrs-nancy.fr)¹

¹ CRPG-ENSG, 12 rue Notre Dame eds Pauvres, 54501 Vandoeuvres-les-Nancy, FRANCE

² ENSG, Rue du Doyen Marcel Roubault, 54501 Vandoeuvres-les-Nancy, FRANCE

During hydraulic-fracturing experiments, a micro-seismic activity has been observed at Soultz, France (HDR Geothermal Site) at a depth ranging from 2800 m to 3600 m. Several thousand seismic events have been induced in a spatial domain with a spread of 1000 m around the well. This seismic information is used to estimate a hydraulic diffusivity of the rock medium surrounding the reservoir. An increase of the pore pressure caused by fluid injection changes the effective normal stress as well as the friction coefficient of the fractured rock mass and the slip induced in the fractures lead to a microseismic triggering. The fractured rock medium is approximated to an effective homogeneous anisotropic poroelastic fluid saturated medium. The triggering front of the hydraulic-induced microseismicity is assumed to propagate like the low-frequency second-type compressional Biot wave, corresponding to the pore pressure relaxation process. Therefore, an estimation of the hydraulic diffusivity tensor is obtained directly from phase-shift information, considering the distances of the events from the center of the injection interval as a function of the event occurrence times. The hydraulic diffusivity, proportional to the permeability is used to evaluate preferential fluid flow directions during the experiment, which is vertical at Soultz. The permeability values obtained are in good agreements with those measured on the wells before fracturing experiments, and with the subvertical orientations tendency of the fractures observed on cores collected on EPS1. Moreover, a second injection test performed on the same well provides new microseismic data which has been used to estimate the increase in permeability after injection test. Such a permeability value is used in a 3D model of the geothermal doublet in order to model the fluid flow during circulation test, and the induced heat transfer. A grid support has been built using the GOCAD software to represent the equivalent porous medium, and to perform in 3D the calculations using the THERMASS finite element code. These simulations intend to predict the necessary time for cooling the rock mass during circulation experiments, that is, to estimate the life time of the natural heat exchanger of the geothermal site at Soultz-sous-Forets.

J05 : 5A/09 : G6

High-Resolution Airborne Geophysical Survey of French Brittany ­ Application to Radiological Risk Monitoring

Catherine Truffert (c.truffert@brgm.fr),

José Perrin (j.perrin@brgm.fr) &

Fouzia Asfirane (f.asfirane@brgm.fr)

BRGM, 3 avenue C. Guillemin, 45060 Orlèans, France

BRGM managed a regional geophysical airborne survey project for the French Industry Ministry. The objective of the survey was to provide a geophysical database in order to aid geological mapping, land-use studies, geohasards evaluation, pollution monitoring and groundwater exploration. The survey was flown between July and November 1998 by Sander Geophysics who won the data-acquisition component of the project under a competitive bidding process. The data were recorded aboard two aircrafts flying at a height of 120 m above land of French Brittany. On the entire survey, line spacing was 500 m or 1000 m and locally 250 m and oriented NS; their orthogonal tie-lines were 10 km apart. Under the BRGM supervision of all stages, total magnetic intensity and gamma-ray emission were recorded during this survey. The gamma-ray spectrometric data were acquired with a system composed by a downward looking detector (40-L NaI crystal) and an upward looking detector (8-L NaI crystal) over the full spectrum of 256 channels (from 0.2 to 3 MeV). Close up view of the survey will be present and specific developments specially dedicated to pollution monitoring, due to nuclear test and Chernobyl accident (Cesium) and Radon production will be shown. The Cesium detection needs specific processing of the lower part of the spectrum (peak energy at 662 keV). Preliminary results show that the Cs level (less than 1 kBq/m²) in this region is lower than the detection threshold. The Radon production, coming directly from Uranium disintegration, can be evaluated by conventional spectrometric Uranium measurements. Specific care has been taken during processing for the atmospheric radon removal in Uranium channel. A map showing outcropping geological formations reach in Uranium and structural features from short wavelength magnetic data which could print out fault drain for Radon.

J05 : 5A/10 : G6

Evaluation of RADARSAT and ERS Data for Mapping Runoff Generating Areas

Nicolas Baghdadi (teledetection@brgm.fr) &

Christine King (c.king@brgm.fr)

BRGM, DR/LGT, 3, av. C. Guillemin, BP 6009, Orleans cedex 02, France

Floods are among the major natural disasters affecting landscapes, agriculture and human activity. They also contribute to the erosion and destruction of soils. The runoff monitoring potential from RADARSAT and ERS data has been studied under the European FLOODGEN project (FLOOD risk reduction by spaceborne recognition of indicators of excess runoff generating areas). The goal of this project is to supply for the territorial agricultural administrators observation and simulation tools of hydrological process and its interaction with the agricultural practices which can lead to increased flood risk. The objective of this paper is to examine the potential of C-band SAR for the watershed roughness characterization and the improvement of the runoff coefficient estimation.

This study has been carried out on RADARSAT (Fine and Standard mode images) and ERS data acquired on the Pays de Caux (Haute Normandie, France) during the winter 1998. These images are geocoded and calibrated. In conjunction with SAR overpasses, field campaigns were conducted in order to measure the dielectric and structural properties of the soil (moisture, surface roughness, etc.) as well as the soil characteristics. The backscattering behavior of training sites chosen on the study area is investigated as a function of polarization, incidence angle, roughness parameter and soil moisture. The agreement between the backscattering coefficients calculated from SAR images and those estimated from the IEM model is also examined. It offers a key of assessment of rate of smoothest surfaces able to favour runoff at the scale of elementary watersheds.

J05 : 5A/11 : G6

Expertise of Ground Lightning Impact Using Remanent Magnetization

Violaine Verrier (verrier@cerege.fr) &

Pierre Rochette (rochette@cerege.fr)

CEREGE Europole de l'Arbois, BP80, 13545, Aix-en-Provence cedex 04

Most of natural materials (rocks, soils, sediments, wood) or artificial materials (concrete, brick, plaster, plastic, glass) contain ferromagnetic microparticles carrying a "natural" remanent magnetization (NRM). This NRM, acquired in the weak geomagnetic field during cooling deposition or solidification of the material, is much weaker than the isothermal remanent magnetization (IRM) acquired instantaneously by exposition to a strong magnetic field.

In the environment, only lightning is able to generate a field strong enough to create an IRM said "lightning induced" (LIRM). The LIRM is stable through time after the lightning impact provided that the material is coherent. This phenomenon is well known (e.g. Graham, 1961) but have never been used to map the lightning peak field Bf around an impact point. We designed an original rock magnetic method to retrieve both the direction and intensity of Bf. The lighning fields recorded can vary between 10 and 100 mT. From the direction and the intensity of Bf the Ampere or Biot and Savart laws permit to estimate the lightning current geometry and its intensity. On a schoolyard tree struck by a lightning we were able to locate the impact point within a few centimeters, proove that it was a vertical positive current and estmate its peak value at 122 kA. Applications of this technique are numerous from the hazard assesment to the understanding of local magnetic anomalies from field magnetic survey which can be of lightning origin. A calibration of peak currents provided by lightning detection networks can be expected.

Graham KWT, Geophys. J. Roy. Astron . Soc, 6, 85-102, (1961).

Session J05:5P

J05 : 5P/01 : PO

Inversion of Slingram Electromagnetic Data for Alluvial Sediments Mapping

Jean Christophe Gourry (jc.gourry@brgm.fr)

BRGM, PO Box 6009, 45060 Orleans, France

The objective of the BRGM' "Morphogenese fluviale" project is to reconstruct fluvial dynamic of the Middle Loire during Holocene in respect with climatic variations and anthropic actions (Garcin et al., 1999). The first topic of this project was to map the alluvial sediments on a 8 km² test zone, 20 km north of Blois, on the Right Bank of the Loire. Thus geophysical methods were used to join the results of the geological descriptions from boreholes.

Electrical soundings carried out 10 years ago have shown that electrical properties of the sediments in this area are highly contrasted : 10 to 20 ohm.m for clayey sediments to 1000 ohm.m for sandy deposits. Limestone substratum resistivity is 60 ohm.m. Sets of 40 km Slingram electromagnetic profiles were carried out in 2 perpendicular directions with the Geonics EM31 and EM34 (20 m spacing) tools. Spacing between profiles was around 150 m. A kriging interpolation yield 2 maps of conductivity.

First, the limits of 2 electrically contrasted bodies were outlined. They were built during two phases of sedimentation. Second, many conductive lineaments are also visible on the maps. They correspond to paleochannels and beds of the Loire which are constituted with clayey or loam sediments.

After this lateral extension description of alluvial sediments, the next step was the vertical geometry of the deposits. Whereas depths of penetration are different, the degree of correlation between the maps is high. Nevertheless, a 3-layers modelling (silt, sandy/clay and limestone) was used with the 2 electrical conductivity as input data. Some a priori constraints were necessary to inverse the data. 14 electrical soundings carried out after the electromagnetic surveys have proved that silt and limestone resistivities are relatively constant, while silt thickness, clayey/sandy layer resistivity and thickness are highly variable. Thus the 3 last parameters were chosen as variables for the evaluation of the non-linear equations. Finally a 3-dimensions lithologic and geoelectric estimation of the alluvial sediments was calculated. It gave good agreements with the results from boreholes.

Garcin M, Giot D, Farjanel G, Klopmmann W, Negrel Ph, Gourry J.C., Irribaria R, J. Conf. Abs., 4, (1999)

J05 : 5P/02 : PO

Application of Experimental Scanning Technology for Study of the Upper Part of a Geological Section

Nathalie Verkhovtseva (Nathalie@ggd.nsu.ru) &

Igor Eltsov (Eltsov@uiggm.nsc.ru)

Institute of Geophysics, pr.Koptuga, 3, Novosibirsk, Russia

Sounding of geological environment by non-stationary and harmonic varying electromagnetic field are the most widely distributed methods of modern geoelectric science and represent the effective method for study of electromagnetic properties of substance. These methods, unfortunately, do not admit the selective analysis of separate layers of geological space. The signals recorded on surface have the integrated nature, and it's difficult to extract the signal from the measurements data, which bears the information about interesting for researcher range of depth. Institute of Geophysics SB RAS develops the new technology for scanning of geological section based on controlled spatially - temporary spectra. Really, changing the geometrical parameters of the unit sounding (sizes of generating and receiver lines, loops and distances between them), is possible to operate spatial spectrum of system of monitoring and, thus, to change sensitivity of signal to elec-trical parameters of geological environment. And changing the form of current pulse of source, we can provide influence on the temporary spectrum of monitoring system, thus achieving the serial predominance in recorded signal of information from different areas of section. Work investigated the applications of new technologies to decisions of tasks of near-surface geoelectricy. For several of typical models of geological environment the opti-mum forms of current pulse and geometrical parameters of sources and receivers were selected. The comparison of results with standard systems of monitoring in the frequent soundings methods and TEM soundings of field was conducted. Noise stability and power-generating efficiency of standard methods of geoelectricy was analyzed. The researches show, that in many cases we can construct the systems of monitoring, efficiency of which considerably exceeds the opportunities of standard methods of geoelectricy.

J05 : 5P/03 : PO

Identification and Correlation of Structural and Sedimentological Discontinuities in Clays Using High Resolution Seismic

Juan Plaza (JPLH@enresa.es),

Patrick Renoux,

Eric Gillot &

Ramon Eptisa

The clayey formation, subject of this presentation, was first identified and localised on the basis of the subsoil information from the hydrocarbon exploration previously carried out in the region.

The clayey formation in enclosed in the stacking of subhorizontal Tertiary materials, deposited in a continental basin of the endorheic type, the performance of which is halfway between that of an intramountainous basin and of a flexion.

The materials of this formation are strategically placed in the middle of the Lower Miocene sedimentary cycle (positive, decreasing grain size) on gradual and concordant transition from the lower terrigenous unit (silts, clays, and sands) of this cycle to the marl-gypsum facies lying on top. There, in turn, it is possible to distinguish sequences of a minor order separated by paraconformities, which coincide with seismic reflectors.

As a second step, in the area where the clayey formation presents of a thickness of over 200 m and without outcropping, seismic reflection and high resolution profiles, continuous control drilling, hydraulic and geotechnical tests were formed as well as detailed geological studies.

The control sample study and the comparative analysis of the drilling logs made it possible first to distinguish two (lower and upper) members in the formation in terms of their U content through the CGR record and a series of sections or areas in each (4 and 5, respectively) and also to define their lithological and petrophysical characteristics.

The levels of clay in this formation, which is largely greenish-grey towards the top, presents silt and marl fine interbeds, sometimes have crystals as well as gypsum and anhydrite nodules in which, from a mineralogical viewpoint, smectite and illite are predominant. These levels correspond to fluvial-lacustrine facies in lateral transit to carbonated or sulphated lacustrine facies, depending on the members and the input direction.

The correlation of the geophysical logs performed in the drillings, supported by the seismic profiles, made it possible to define the spatial evolution and geometrical variations as well as the sedimentological and therefore petrophysical characteristics of the different members and section identified in the formation.

Using normal velocity processing and specific acoustic impedance processing (Rovin) in the executed profiles, the seismic inversion has also made it possible to visualise and locate the lithological changes, identify the different types of structural discontinuities (compensated faults, isolated fractures, etc.), apart from delimiting their position, continuity and frequency.

Finally, by integrating all the information obtained, it was possible spatially to delimit the distribution of the lithological units of which the clayey formation consists as well as the structural discontinuities by which it is affected, thus distinguishing the areas with different lithological and structural characteristics.

J05 : 5P/04 : PO

Rock Destruction Effect: A Novel Physical-Mathematical Approach

A. L. Aleinikov¹,

V. T. Belikov² &

L. V. Eppelbaum (lev@jupiter1.tau.ac.il)³

¹ Geophysical Consultant, 30/9 Mapiley Egoz, Tel Aviv 67437, Israel

² Inst. of Geophysics, Ural Branch of RAS, 100 Amundsen St., Ekaterinburg, Russia

³ Dept. of Geophysics, Tel Aviv Univ., Ramat Aviv, 69978, Tel Aviv, Israel

Investigation of mountainous rock destruction under different thermodynamic conditions is one of the most important problems of engineering hazard. It is generally considered that destruction is taking place when at least one component of the stress tensor reaches some critical value called as a material strength. However, conducted analysis allowed us to conclude that such a presentation is correct only for a relatively short-term action and is unsuitable by a prolonged stress to object.

It was earlier empirically determined (Zhurkov, 1968) that increasing of time action stress (<tau>) cause destruction of rocks by lesser values of strength:

(1)

where <alpha> is the applied stress, <Gamma> is the absolute temperature, K is the Boltzmann constant; <tau>₀ is the parameter coinciding with the period of oscillation of atoms in medium; u₀ is the energy of activation destruction in the initial non-stressed condition, close to the sublimation energy; <gamma> is the structural factor indicating volume of the stress zone concentration. By destruction of the laboratory samples has been discovered the presence of distracted material in the gaseous state, i.e. as a new phase. Considering the destruction process as a phase transfer, the relationship between time <tau> and stress (<sigma>) values may be expressed as

(2)

where <lamda>_ik is the Kronecker's symbol; P' is the steam pressure in the area where the crystalline grating is distracted; <sigma>_ik and u_ik are the tensions of stress and deformations, respectively.

It is obvious that by one-fold expansion and P'=0 equation (2) is no different from the empirically determined relationship (1). Thus, on the basement of equation (2) may be developed procedures for estimating a possible duration of different artificial underground constructions: mines, reservoirs, etc.

Generation of microfaults in medium initiates emission of seismo-acoustical impulses. Quantity of the impulses during the time unit shows an intensity of process destruction and oscillation frequency indicates dimension of the forming microfaults:

(3)

where <nu>⁽¹⁾ is the oscillation frequency; <rho> is the density; <Omega> is the specific surface of the heterogeneous medium; L_o^<alpha> is the dimension of non-deformed <alpha>-phase and f_<alpha> is the elastic modulus of the phase.

Measuring the intensity of seismo-acoustical emission and quantity of gases extracting by fault generation may be used as an effective mean for detecting dangerous areas in the stress-deforming media.